1. Technical Field
The present subject matter relates to a method for reading license plate numbers by means of optical character recognition (OCR) in a road network.
2. Background Art
Methods for reading license plate numbers by means of OCR in a road network are used, e.g., for video-based road toll systems or parking fee systems in order to identify vehicles based on their license plate number (LPN), and thereby to levy fees for vehicles using a location, to track them, or to impose fines for using a location. In image recordings of the license plates of passing vehicles, the license plate numbers are localized and converted by an OCR reading process into a character string of individual characters.
However, due to changing lighting conditions and changing visibility, e.g., due to snow, rain, shadowing effects or glares, and also due to soiling or disadvantageous mounting of the license plate, the error rate of OCR methods in practical operation is very high, in most cases 10-15%. This problem becomes even more serious due to the fact that the installation locations of the OCR reading stations are often predetermined by legal or technical framework conditions, e.g., at border stations or places where power and data connections are available, and therefore cannot be selected with regard to advantageous visibility, which further complicates capturing proper images of license plate numbers and thus makes the OCR process more difficult.